The invention relates to projection display apparatus employing a laser as a light source. More particularly, the invention relates to laser projection display apparatus having means for reducing the appearance of coherence-induced artifacts and speckle in the display.
Projection display systems for the display of video images are well-known in the prior art. Typically, these systems have taken the basic form of a white light source, most notably a xenon arc lamp, illuminating one or more light valves or spatial light modulators with appropriate color filtering to form the desired image, the image being projected onto a viewing screen.
Lasers have been known to be attractive alternative light sources to arc lamps for projection displays. One potential advantage is a wider color gamut featuring very saturated colors. Laser illumination offers the potential for simple, low-cost efficient optical systems, providing improved efficiency and higher contrast when paired with some spatial light modulators. One disadvantage of lasers for projection display has been the lack of a cost-effective laser source with sufficient power at visible wavelengths.
Spatial light modulators provide another component that enables laser display systems. Examples of two-dimensional spatial light modulators are reflective liquid crystal modulators such as the liquid-crystal-on-silicon (LCOS) modulators available from JVC, Three-Five, Aurora, and Philips, and micromirror arrays such as the Digital Light Processing (DLP) chips available from Texas Instruments. Advantages of two-dimensional modulators over one-dimensional array modulators and raster-scanned systems are the absence of scanning required, absence of streak artifacts due to nonuniformities in the modulator array, and immunity to laser noise at frequencies much greater than the frame refresh rate (xe2x89xa7120 Hz). A further advantage of two-dimensional spatial light modulators is the wide tolerance for reduction of the spatial coherence of the illuminating beam. Examples of one-dimensional or linear spatial light modulators are the Grating Light Valve (GLV) produced by Silicon Light Machines and conformal grating modulators; see U.S. Ser. No. 09/491,354, filed Jan. 26, 2000, by Kowarz, and U.S. Ser. No. 09/867,927 filed May 30, 2001, by Kowarz et al.
Although high power visible lasers offer new opportunities for the design of projection systems, including the possibilities of expanded color gamut and simplified optical designs, laser light is in other ways not optimum for use in image projection systems with spatial light modulators. In particular, lasers are very bright sources, which emit generally coherent light within a very small optical volume (etendue or lagrange). Etendue is the product of the focal spot area and the solid angle of the beam at the focus. Lagrange is the product of the focal spot radius and the numerical aperture. For example, a single mode green wavelength laser with a diffraction-limited beam has a lagrange of about 0.3 xcexcm, which is about 15,000 times smaller than the lagrange for a conventional white light lamp source, such as an arc lamp. With such a small lagrange, lasers can be used very effectively in raster scanning systems, including those for flying spot printers and laser light shows, where a tightly controlled beam is desirable.
On the other hand, in an image projection system, in which an image-bearing medium such as a film or a spatial light modulator is imaged to a screen or a target plane, the high coherence and small lagrange of the laser is ultimately undesirable. In such an imaging system, the lagrange is determined by the linear size of the projected area (size of the spatial light modulator) multiplied by the numerical aperture of the collection lens. The related quantity, etendue, is calculated similarly. In many white light projection systems, the projection lens is quite fast (f/3 for example) to collect as much light as possible. Even so, the typical white light lamp source overfills both the light valve and the projection lens, and significant light is lost. For example, in a representative system using a common 0.9xe2x80x3 diagonal light valve and an f/3 projection lens, the optimum light source would have approximately a 2.0-mm lagrange to provide proper filling without overfill. However, a standard white light lamp, with a typical lagrange of 2-10 mm, is not sufficiently bright and will generally overfill this representative system.
In the case of a laser display system using image area projection, the opposite problem arises, the lasers being too bright. Furthermore, it is not desirable to illuminate the spatial light modulator with a coherent source, because of the potential for interference effects, such as fringes, which may overlay the displayed image. This is especially true of liquid crystal modulators, wherein the thin-film structure can result in fringes in the image due to nonuniformities in the film layers. Likewise, coherent illumination of the entire light valve may create an artificial beam apodization, which is dependent on the image content, arising from diffraction from pixels and groups of pixels. Diffraction artifacts can also arise from illuminating the grid electrode pattern of a liquid crystal panel, an X-cube with a center discontinuity, or any dust or imperfections on the optical elements with a highly coherent beam of light. Therefore, a reduction of the source brightness (or an increase in the source lagrange) is a necessity for such laser projection systems.
A defined reduction of the source brightness can also provide an important opportunity. The projection display optical system can be designed to optimize and balance the system requirements for resolution, system light efficiency, and system simplicity. By defining the system f-number on the basis of a criterion other than system light efficiency, the specifications on other system components such as the projection lens, color filters, and polarization optics can be eased, dramatically reducing system costs compared to lamp-based projection systems.
While laser sources can be optimized for use in projection display illumination and imaging systems, there is the consequent major disadvantage of speckle to be dealt with. Speckle arises due to the high degree of coherence (both spatial and temporal) inherent in most laser sources. Speckle produces a noise component in the image that appears as a granular structure, which both degrades the actual sharpness of the image and annoys the viewer. As such, the speckle problem, as well as the historical lack of appropriate laser sources, has inhibited the development of marketable laser-based display systems.
The prior art is rich in ways of attempting to reduce speckle. One common approach is to reduce the temporal coherence by broadening the linewidth of the laser light. Other approaches to reducing the temporal coherence are to split the illuminating wavefront into beamlets and delay them relative to each other by longer than the coherence time of the laser, see for example U.S. Pat. No. 5,224,200, issued Jun. 29, 1993 to Rasmussen et al. Dynamically varying the speckle pattern by vibrating or dynamically altering the screen is another way of reducing the visibility of the speckle pattern. See, for example, U.S. Pat. No. 5,272,473 issued Dec. 21, 1993 to Thompson et al. Another speckle reduction approach involves coupling the laser light into a multimode optical fiber and vibrating the fiber to cause mode-scrambling as described in U.S. Pat. No. 3,588,217, issued Jun. 28, 1971 to Mathisen.
Another family of despeckling solutions uses a diffusing element that is moved or vibrated within the projector system. Typically, this is done at an intermediate image plane, as disclosed in U.S. Pat. No. 4,035,068, issued Jul. 12, 1977 to Rawson. One disadvantage of this approach is that the diffusion must occur precisely at the image plane or a softening of the image will occur. Also, the projection lens is complicated by the requirement to provide an intermediate image plane. A means of dynamically varying the speckle pattern by dynamically diffusing the laser beam in the illumination path of the device would be preferable. A hologram illumination system utilizing this approach has been disclosed by van Ligten in U.S. Pat. No. 3,490,827, issued Jan. 20, 1970, in which a diffuser is rotated in the focus of a beam expander. Florence discloses in U.S. Pat. No. 5,313,479, issued May 17, 1994, illuminating a light valve through a rotating diffuser. These approaches have the disadvantage of not being adaptable to uniform efficient illumination of a rectangular spatial light modulator. Butterworth et al. in U.S. Pat. No. 6,005,722, issued Dec. 21, 1999, disclose a system in which a variable-thickness plate is rotated in the illumination of a lightpipe homogenizer. When used with lasers, though, light pipe homogenizers require either a large numerical aperture or a substantial length to achieve sufficient uniformity, and offer less control with fewer degrees of design freedom than systems designed with fly""s eye optics. Therefore, it is harder to control the illumination brightness while producing a uniform illumination in a compact system. Furthermore, modeling performed by the present inventors suggests that in a long-throw projection system, the diffuser should be placed near a position conjugate to the screen in order to reduce speckle. This condition is not achieved in the system disclosed by Butterworth et al, wherein the diffuser is at the input of a light pipe.
There is a need therefore for a laser-based display system that uses a spatial light modulator, allows control of the illumination brightness to optimize system design, and exhibits reduced speckle and eliminates coherence artifacts at the spatial light modulator while exhibiting high throughput efficiency.
The need is met by providing a display apparatus that includes a laser light source for emitting a light beam; a beam expander for expanding the light beam; a spatial light modulator; beam shaping optics for shaping the expanded laser beam to provide uniform illumination of the spatial light modulator, the beam shaping optics including a fly""s eye integrator having an array of lenslets; and a moving diffuser located in the laser beam between the laser light source and the spatial light modulator.
The display device of the present invention has all of the advantages of a laser display system, including high brightness, and saturated colors, with substantially reduced speckle. It also removes the speckle in such a way that neither image quality nor illumination efficiency is compromised, in contrast with other despeckling techniques using diffusers. Further, the invention allows for the control of the illumination lagrange (or etendue) in order to optimize the design of the optical system.